Boron nitride boat and process for producing it

ABSTRACT

A pyrolytic boron nitride boat having a cavity suitable for use in growing and doping semi-conductor crystals such as gallium arsenide and said cavity having a roughened surface formed of substantially uniform projected no nodules, disturbances, or ridges.

This application is a division of prior U.S. application Ser. No.07/516,910, filing date Apr. 30, 1990 which is now U.S. Pat. No.5,032,366.

FIELD OF THE INVENTION

This invention relates to an improved boron nitride boat having a cavitywith substantially uniform projected nodules that make the boat ideallysuited for use in growing and doping semiconductor crystals, such asgallium arsenide (GaAs) crystals.

BACKGROUND OF THE INVENTION

The structure, physical properties, purity, and chemical inertness ofpyrolytic boron nitride (PBN) make it an attractive container materialfor elemental purification, compounding, and growth of semi-conductorcrystals. Examples include containers for liquid-encapsulatedCzochralski (LEC) and vertical gradient freeze (VGF) growth of GaAs andother III-V and II-VI compound single crystals, and source containersfor deposition of metals and dopants at high temperatures and ultra-highvacuum by molecular beam epitaxy (MBE). Recently, pyrolytic boronnitride has been used as a container for growth of GaAs crystals by aliquid encapsulated vertical zone melting process. GaAs crystals withextremely low carbon content have been produced in liquid-encapsulatedCzochralski furnaces where the graphite furnace parts were coated withpyrolytic boron nitride.

Pyrolytic boron nitride can be produced by various methods such as themethod disclosed in U.S. Pat No. 3,152,006, in which pyrolytic boronnitride is produced by the vapor-phase reaction of ammonia and boronhalides, such as boron trichloride. By depositing the boron nitrideproduced in this manner upon a suitable mandrel, such as a graphitemandrel, a wide assortment of shapes can be produced. Boats of pyrolyticboron nitride have been produced in this manner with smooth surfacesthat may be wetted by a molten element or compound. As used herein,wetting shall mean a physical surface reaction between a molten materialand the surface of a boat which results in sticking and/or adhesion ofthe molten material on the surface. Wetting results in internal stressduring solidification of the molten material (for example, an element orcompound) and as a consequence, imperfect product may be produced. Forexample, when producing single crystals in smooth surface boats, thewetting of the walls by the molten material can cause internal stressduring solidification which can result in imperfect crystal formation,i.e., polycrystal or twinning.

Most gallium arsenide single crystal is grown in quartz horizontalboats. This material is characterized by its low electrical resistanceand high degree of crystal perfection. The low electrical resistance isgenerally a result of silicon contamination (autodoping) from the quartzboat. The level of autodoping is generally non-uniform along the boatlength with the lowest level at the seed end. This autodoping of siliconfrom the quartz boat limits the crystal produced to applications notrequiring high electrical resistance, i.e., light emitting-diodes. It isdisclosed in the literature that twinning can be reduced by sandblastingthe surface of the cavity of a quartz boat in which the single crystalis grown.

Elimination of silicon autodoping would improve electrical propertiesuniformity of these crystals and expand their use to applicationsrequiring a high electrical resistance, i.e., integrated circuits.Pyrolytic boron nitride boats have been used to produce undoped highresistance gallium arsenide crystals in which the silicon contaminationwas effectively eliminated but, however, the formation of single crystalgrowth was hindered because of a surface reaction (wetting) between thegallium arsenide melt and the surface of the pyrolytic boron nitrideboat. This interface problem cannot be overcome by sandblasting thepyrolytic boron nitride boat because of the laminar nature of thepyrolytic boron nitride structure. The thickness of the pyrolytic boronnitride lamina is generally too thin (about 0.5 mil thick) to allowsufficient surface roughness to occur before the entire lamina isremoved and a fresh, smooth lamina exposed.

It is an object of the present invention to provide a process forproducing a pyrolytic boron nitride boat having a cavity with a nodularsurface that is ideally suited for use in the growth of single crystalmaterials.

It is another object of the present invention to provide a process forproducing a pyrolytic boron nitride boat having a cavity with a nodularsurface that is ideally suited for the growth of gallium arsenide.

It is another object of the present invention to provide a pyrolyticboron nitride boat having a cavity with a nodular surface that isideally suited for the growth of gallium arsenide.

The foregoing and additional objects will become fully apparent from thefollowing description and the accompanying drawings.

SUMMARY OF THE INVENTION

The invention relates to a process for producing a boron nitride boatwith a cavity in which at least a portion of the cavity has asubstantially uniform distribution of projected nodules, said processcomprising the steps:

(a) preparing a mandrel having the shape of a desired boat to beproduced and treating at least a portion of the mandrel, thatcorresponds to the cavity of the boat, to form a substantially uniformdistribution of projected nodules on said portion of the mandrel andwherein said nodules have an average height of at least one mil; and

(b) depositing boron nitride upon said mandrel of step (a) until thedesired thickness of boron nitride is deposited on said mandrel, andremoving the boron nitride boat from the mandrel, said boat having acavity with a substantially uniform distribution of projected noduleshaving an average height of at least one mil.

The boron nitride could be deposited by reacting ammonia and a boronhalide in a vapor phase at a temperature of from about 1450° C. to about2300° C. under a pressure no greater than 50 mm, preferably no greaterthan 1 mm, of mercury absolute.

Generally, deposited boron nitride has a laminar structure and thelaminae, which are stacks of crystalline basal planes, are between about0.5 to 1.5 mils thick and typically about 0.5 mil thick. The height ofthe projected nodules on the surface of the boat shall preferably bemore than the thickness of an individual lamina of the deposited boronnitride.

As used herein, boats shall mean crucibles or any other container whichcan be used in the art for single crystal growth.

As used herein, the term projected nodules shall mean a textured surfacewhich has a plurality of uniform or non-uniform disturbances or ridgeswhich project above the lowest plane of the surface. The disturbancesshould be at least one mil high and preferably about 1.5 to 3 mils high.Preferably, the height of the projected nodules or disturbances shouldbe at least two times larger than the thickness of a lamina of thedeposited pyrolytic boron nitride. As stated above, althoughsandblasting techniques can be used to provide a nodular surface,pyrolytic boron nitride has a laminar structure composed of smooth thinlaminae and therefore sandblasting would essentially remove a laminaonly to expose an under lamina which would be a smooth surface. Toovercome this problem, the mandrel is prepared with a nodular surface sothat as the pyrolytic boron nitride is deposited on the surface of themandrel, each lamina will follow the contour of the nodular surface.This will result is a boat having a cavity composed of thin laminae eachof which has a nodular contour. Thus even if a pyrolytic boron nitridelamina is removed, the same nodular or textured surface will exist inthe exposed new lamina thereby permitting multiple uses of the pyrolyticboron nitride boat. Generally, a conventional quartz boat is limited toone crystal growth cycle because of softening and deformation, sticking,vitrification or cracking. The novel pyrolytic boron nitride boat ofthis invention has a cavity with a nodular surface that is resistant tocracking and has non-sticking characteristics. Since the novel pyrolyticboron nitride boat of this invention is composed of a plurality oflaminae, with each lamina having a nodular contour, the wearing away ofthe top lamina will expose a lamina having a similar nodular surfacethus making the boat suitable for reuse without destroying the nodularsurface characteristics of the boat. In addition, the pyrolytic boronnitride boat of this invention is believed to have the followingbenefits:

1) permits single crystal growth with higher yields than non-nodularsurface pyrolytic boron nitride boats;

2) reduces production costs as compared to use of quartz boats;

3) allows production of undoped semi-insulating GaAs single crystalswith low defect density; and

4) allows production of more uniformly doped semi-conducting GaAscrystals with low defect density.

The mandrel, preferably made of graphite, can be made with a nodularsurface by sandblasting, machining, distressing or fabricating themandrel with coarse or porous graphite. At least the surface of themandrel corresponding to the cavity of the boat shall be treated orfabricated with a nodular surface.

In order to produce the boats of the present invention, the boronnitride is deposited upon a mandrel having the same shape as the desiredboat and the surface area on the mandrel corresponding to the cavity ofthe boat should have a nodular surface. The mandrel employed, of course,must be one which does not melt at the temperature at which the boronnitride is applied and which is inert to the boron halide and ammonia atsuch temperature. Generally, the mandrel employed is made of graphite.

Typically, the mandrel upon which the boron nitride boat is to be formedis mounted in a vapor deposition furnace and, after the furnace isheated to the desired temperature, the ammonia and boron halide gas,generally boron trichloride, are introduced into the reactor. Thereaction between the ammonia and boron halide, and deposition of theboron nitride produced by this reaction, is typically effected at atemperature of from about 1450° C., to about 2300° C., and the reactoris accordingly maintained within this range. Preferably the temperatureof the reactor is maintained between about 1850° C. and 1950° C.

The reactants are introduced into the reactor in vapor phase. Generally,at least 1 mole of ammonia is employed per mole of boron halide, with anexcess of ammonia being preferred. Most preferably, from 2.5 to 3.5moles of ammonia are employed per mole of boron halide, although evengreater excesses can be employed if desired. The flow rate of thereactants through the reactor depends upon the specific design of thereactor and the size and shape of the mandrel upon which the boronnitride is to be deposited. Generally, flow rates of from about 0.2standard cubic meters/hour to about 0.3 standard cubic meters/hour ofammonia and from about 0.06 standard cubic meters/hour to about 0.1standard cubic meters/hour of boron halide per 1.5-2.5 cubic meters offurnace volume are suitable. If desired, an inert gas may be intermixedwith the reactant gases.

After a suitable time, i.e., after the desired amount of boron nitridehas been deposited on the mandrel, the flow of reactants into thereactor is interrupted and the reactor is cooled to room temperature.The pyrolytic boron nitride boat can then be removed from the mandreland, if necessary, cut to a desired length and cleaned.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an isometric view of a boat of this invention for growingsingle crystals.

FIG. 2 is a greatly enlarged cross-sectional view of FIG. 1 takenthrough line A--A showing the laminated structure of the pyrolytic boronnitride material.

FIG. 2a is an enlarged fragmentary view of the laminated structure ofthe pyrolytic boron nitride material of FIG. 2.

FIG. 3 is a greatly enlarged cross-sectional view of a conventionalpyrolytic boron nitride boat showing the laminated structure of thepyrolytic boron nitride material.

FIG. 1 shows a pyrolytic boron nitride boat 2 having a cavity 4 andupstanding wall 6. The internal surface 8 of cavity 4 has a nodularsurface which will reduce the tendency of the crystal melt in saidcavity 4 to wet the surface while also reducing the drag that causesstress on the crystal melt during solidification. The use of a pyrolyticboron nitride boat 2 with a nodular surface 8 will improve the yield ofsingle crystals. FIG. 2 is a cross-section of the pyrolytic boronnitride boat 2 of FIG. 1 greatly enlarged to show the laminatedstructure of the pyrolytic boron nitride. Specifically, the pyrolyticboron nitride boat 2 is comprised of a plurality of laminae 10 eachhaving a nodular surface 12. This laminated structure is formed bydepositing pyrolytic boron nitride onto a mandrel having a nodularsurface on at least the area corresponding to the cavity 4 of boat 2. Asshown in FIG. 2a, the projected nodules 3 have a height D which ismeasured from the plane of the lowest point 5 as shown by the brokenlines to the top of the projected nodules 3. The height of the nodulesshould preferably be greater than the height of an individual lamina ofthe deposited pyrolytic boron nitride laminated structure, and mostpreferably greater than two times the thickness of the individuallamina. The laminar nature of the deposited pyrolytic boron nitridestructure results in individual thin lamina being deposited on themandrel and all such laminae have the nodular surface contourcorresponding to the nodular surface of the mandrel. As stated above,even if a portion of the top lamina is removed, the same textured ornodular contour exist in the newly exposed lamina thus permittingmultiple uses of the pyrolytic boron nitride boat without requiringfurther surface preparation.

FIG. 3 shows an enlarged cross-section of a conventional pyrolytic boronnitride boat 14 comprised of a laminated structure. The pyrolytic boronnitride boat 14 is comprised of a plurality of smooth laminae 16. Thelaminae 16 of the conventional pyrolytic boron nitride boat 14 usuallyis about 0.5 mil thick. Thus to impart a nodular surface to aconventional pyrolytic boron nitride boat by sandblasting would beineffective since the laminae of the pyrolytic boron nitride are toothin to allow sufficient surface roughness to occur before the entiretop lamina is removed and a fresh, smooth lamina is exposed. Contrary tothis, the pyrolytic boron nitride boat of the subject invention iscomprised of a plurality of laminae each having a nodular contoursurface. In addition, it is believed that sandblasting of a pyrolyticboron nitride boat could expose the edge planes of the deposited laminaof the boron nitride and these edges are believed to be more wettablethan the basal planes produced by the boron nitride deposition.

EXAMPLE

A graphite mandrel was produced having a shape of the desired boat. Thegraphite mandrel after machining was sandblasted with 60 mesh (0.42 mm)Al₂ O₃ grit at 2 kg/cm² air pressure. The surface of the graphitemandrel was held about 20 to 25 cm from the nozzle of the sandblastermachine. The mandrel was mounted in a 0.33 cubic meter vapor depositionfurnace. The pressure in the furnace was reduced to 0.5 mm of mercuryabsolute and the temperature was raised to 1900° C. Gaseous borontrichloride and ammonia were then introduced into the reactor. The flowrate of the ammonia through the reactor was 1.5 standard cubicmeters/hour, and the flow rate of the boron trichloride was 2.0 standardcubic meters/hour. After 40 hours of operation, the flow of ammonia andboron trichloride was stopped. After the reactor had cooled to roomtemperature, the multi-laminated pyrolytic boron nitride boat was easilyremoved from the graphite mandrel and had a nodular surface in thecavity.

The pyrolytic boron nitride boat of this invention is believed to bebeneficial to produce GaAs single crystals using conventionaltechnology. It is also believed that the boat of this invention willpermit single crystal growth with higher yields than conventionalnon-nodule pyrolytic boron nitride boats.

It is to be understood that although the present invention has beendescribed with reference to many particular details thereof, it is notintended that these details shall be construed as limiting the scope ofthis invention. For example, the boat of this invention could comprise agraphite boat that is treated to have a nodular surface and then thepyrolytic boron nitride could be vapor-deposited onto the graphitesubstrate to yield a pyrolytic boron nitride boat having a nodularsurface at least in the area defining the cavity.

It is also within the scope of this invention to use a female mandrel ormale mandrel to make the boron nitride boat. Preferally, a male mandrelwould be used to produce the boron nitride boat.

What is claimed is:
 1. A pyrolytic boron nitride boat having a cavityand said pyrolytic boron nitride comprising a multi-laminated structureof boron nitride in which each lamina of the multi-laminated structureof the boron nitride defining said cavity has a nodular surface andwherein said nodular surface of the top lamina is composed of asubstantially uniform distribution of projected nodules in which theaverage height of the projected nodules is at least two times greaterthan the thickness of the boron nitride top lamina.
 2. The pyrolyticboron nitride boat of claim 1 wherein the average thickness of eachboron nitride lamina forming the multi-laminated structure and having anodular surface is about 0.5 mil thick.
 3. The pyrolytic boron nitrideboat of claim 1 wherein the average height of the projected nodules isabout 1.5 mils.
 4. The pyrolytic boron nitride boat of claim 1 whereinthe multi-laminated structure of boron nitride is disposed over asubstrate having a nodular surface and each lamina of the boron nitridedefining the cavity has a nodular surface.
 5. The pyrolytic boronnitride boat of claim 4 wherein said substrate is graphite.
 6. Thepyrolytic boron nitride boat of claim 4 wherein the average thickness ofeach boron nitride lamina forming the multi-laminated structure andhaving a nodular surface is about 0.5 mil thick.
 7. The pyrolytic boronnitride boat of claim 4 wherein said nodular surface of the top laminais composed of a substantially uniform distribution of projected nodulesin which the average height of the projected nodules is at least twotimes greater than the thickness of the boron nitride top lamina.
 8. Thepyrolytic boron nitride boat of claim 7 wherein the average height ofthe projected nodules is about 1.5 mils.